This invention relates a method and an apparatus for the sterilisation and/or disinfection of packaged articles such as packaged food and drink products.
The shelf life of food is substantially shortened due to the presence of micro-organisms in the food, which can cause the food to deteriorate. Not only does shelf life affect the economic viability of food producers but it has a direct effect on public health, since the presence of certain micro-organisms in food can be hazardous if the food is ingested. These problems can be exacerbated if the food is not kept sufficiently refrigerated, since the micro-organisms in the food can multiply rapidly.
In order to overcome the above-mentioned problems, it has been proposed to pasteurise food. However, a disadvantage of pasteurisation is that the process is lengthy and can only be used on certain types of food. Furthermore, the pasteurisation process affects the taste of the food and is costly to perform, since it uses a substantial amount of energy, a great deal of which is discharged into the working environment. In one known method, the food is packaged in an atmosphere which inhibits the fast reproduction of micro organisms. One such an approach is to package the food product within a carbon dioxide atmosphere. This has proved to be difficult to control, environmentally unfriendly and expensive to run. GB2457057 discloses an alternative method in which the food product is disinfected by irradiating it with UV light through its sealed packaging. This method requires the packaging material to pass the disinfection wavelengths (around 260 nm) at high efficiencies, otherwise high power is required to get sufficient UV intensity into the package to disinfect the food. Present packaging materials are poor transmitters of these UV wavelengths and therefore special packaging materials need to be used. Such packaging materials are expensive and necessitate modifications to the existing packaging processes, which mean that the whole food industry will have to change its packaging equipment or develop a whole new family of packaging materials.
In order to achieve adequate disinfection inside a sealed package it is necessary that all of the product surfaces are irradiated with the UV light. This is extremely difficult to achieve, for example in the case of sliced meat or cheese where the light will not reach between the slices therefore the disinfection effect will be marginal and therefore the shelf life will not be improved. The method also suffers from a susceptibility to dust and dirt, since the UV lamps must be clean at all times and it will be appreciated that the general environment in the food processing industry does not lend itself to this. This method also has the added disadvantage that the UV light must have a clear “window” to penetrate the package i.e. no labelling or printing on the package. This makes the packaging process inflexible and forces packaging process redesign.
It is well known that ozone is a highly oxidising gas, which is a very efficient disinfector of micro-organisms. Ozone has a very short life (about 20 minutes) before it naturally reverts back to oxygen and therefore ideally suited for extending the shelf life of food sold in sealed packages and for killing other harmful micro-organisms that may be contained in the food such as e-coli. 
GB2457057 also discloses a method in which the food product is further disinfected in its sealed package by creating ozone inside the package using UV light of ozone producing wavelengths. Ozone, being a gas with very efficient disinfection properties, will permeate everywhere inside the sealed package and will therefore disinfect the product. Unfortunately this method suffers from the same disadvantages as the above-mentioned UV disinfection method, in that the packaging materials to pass such UV wavelengths are even more special and are expensive to buy and process. Also, the ozone producing wavelengths are in the vacuum UV range (around 185 nm) and known packaging materials pass these wavelengths inefficiently and hence are energy inefficient.
In practice, the amount of ozone produced by UV methods is relatively low and is significantly affected by atmospheric humidity. Accordingly, in a fixed flow process where the time to dose each package is fixed, it is very difficult to get a consistent ozone dose. This method also produces nitrous oxide as a by product from the air inside the package which is undesirable, since nitrous oxide combined with water produces nitric acid which will damage the product. Another drawback to this approach is that there is an amount of unwanted ozone produced in the air spaces surrounding the UV lamp, which must be neutralized as free ozone is a regulated substance because the presence of ozone in the atmosphere presents a health hazard.
This method also has the added disadvantage that the UV must have a clear window to penetrate the package i.e. no labelling or printing on the package. This makes the packaging process inflexible and forces packaging process redesign. Another known method of sterilising food comprises creating ozone inside a sealed package using a conventional corona discharge methods. This entails a metal electrode placed either side of the sealed package and a high voltage ac supply connected to the electrodes. The high voltage creates a corona discharge between the electrodes, which then converts some of the oxygen in the air in the package to ozone.
Whilst this method avoids some of the problems with the UV irradiation method, it still suffers from some serious shortcomings. The method uses metal electrodes, which heat up to a significantly high temperature during operation and therefore need to be force cooled. These electrodes are in close proximity to the packaging material and hence have to be cooled to less than 70 degrees centigrade, otherwise the packaging material is degraded. This usually requires water cooling with its associated pumping and heat exchanger systems. This method is a discharge system, which means that electrons are discharged between the electrodes under high voltage conditions: as a consequence there is erosion and hence deterioration of the electrodes leading to short electrode life and hence poor reliability. Discharges of this technology are uncontrolled avalanche types, which not only penetrate the packaging material but also the product and can be very detrimental to some products. This method usually cannot be repeated more than once as the product deterioration due to repeated corona discharge is unacceptable. Corona discharge whilst producing medium to high levels of ozone also suffers from inconsistent ozone production due to atmospheric humidity and worse produces high levels of nitrous oxide from the nitrogen in the air inside the package. As a consequence this method is usually confined to applications where the packaging environment is pure oxygen and hence no nitrous oxide is formed. To package product in oxygen is both difficult to control and expensive.
Our earlier publication WO2010/116191 provides an advantageous method and apparatus for generating ozone within a package using gas filled electrodes. We have now found that it is possible to provide useful ozone sterilization using elongate solid state electrodes and have provided a number of other improvements to reduce power requirements and increase effectiveness.